The mRNA-based vaccines cannot lead to COVID-19 or its symptoms since they only lead to the production of the spike protein in the cell.
However, the spike protein itself can lead to cell fusion: Quantitative assays reveal cell fusion at minimal levels of SARS-CoV-2 spike protein and fusion from without
Is this a potential risk of the mRNA vaccine ? I could imagine, that the spike protein needs to be bound to the virus to induce this effect ? I could not find any literature where the effect of the unbound spike protein is tested.